In cryogenic refrigerators such as Stirling and Gifford-MacMahon type refrigerators, a piston-like displacer reciprocates within a cylinder. For efficient refrigeration in those two systems, a gas seal is provided between the displacer and cylinder to assure that refrigerant passing from one end of the displacer to the other passes through a regenerator in the displacer. The seal also provides a braking action on displacer movement. One type of seal which has been widely used in such refrigerators is the split ring seal having a Z-cut there across to permit circumferential expansion of the ring without loss of sealing. The seal is positioned in a circumferential groove in the displacer and is pressed outward against the cylinder by an inner expander ring. When assembled, the split seal fits snugly within the groove. A primary advantage of such a seal in cryogenic refrigerators is that, even with wear and thermal contraction or expansion of the seal, the expander assures a fairly constant braking force on the displacer movement.
As shown in FIG. 1, a typical split Gifford-MacMahon or Stirling refrigerator includes a displacer 12 which reciprocates in a cylinder 14. The displacer is driven by a motor or gas spring volume through a piston rod 13 and pin connection 15. Upward movement of the displacer 12 causes high pressure gas in a warm chamber 16 to be displaced through a regenerator 18 within the displacer. The gas passes outward through a porous metal plug 20 or side ports (not shown) into a cold end expansion chamber 24. The thus cooled gas is expanded in the chamber 24 to further cool the gas surrounding high conductivity heat station 26. Then, with downward movement of the displacer the very cold gas is returned through the regenerator 18 to cool that regenerator for cooling of gas in the next cycle of operation. To assure that all refrigerant, such as helium gas, is directed through the regenerator with movement of the displacer, the seal ring 28 is positioned in a peripheral groove in the displacer near its warm end.
The seal ring 30 is generally of plastic material such as fiber glass tetrafluoroethylene (TFE), whereas the expander ring is of spring steel and the groove is usually formed in stainless steel. Thus, the seal ring 30 has a much greater coefficient of thermal expansion than do the surrounding metal parts. To allow for thermal expansion of the seal ring within the groove, rings in conventional refrigerators have a slightly smaller axial dimension than that of the groove to within 0.5 mils. Thus, except when operating at high temperatures, the seal ring shuttles within the groove along the expander ring with each change in direction of the displacer movement. During the time intervals that the seal is shifting within the groove, there is no compressive force to maintain a tight seal along a circumferential cut or Z-cut in the seal ring 30 and at a groove face. Thus, leakage is experienced with each change in direction of the displacer. Also, the seal ring must alternately seal against opposite faces of the groove. For this sealing, the faces of the seal ring and the groove must be precisely smooth, flat and parallel. Such necessary precision, makes large scale fabrication of consistently and uniformly operating devices difficult.
When the refrigerator is exposed to a cold environment, or the seal is cooled by the refrigerator itself, the seal ring shrinks and, as a result, shuttles to a greater extent along the expander ring with each reciprocating movement of the displacer. Such shuttling of the seal within the groove results in greater wear and leakage. The wear of the seal ring leads to even greater leakage around the displacer and also gives rise to debris which, when mixed with the helium refrigerant, reduces the efficiency of the refrigerator.
In U.S. Pat. No. 4,355,519, herein incorporated by reference, shuttling of the seal was prevented by the use of a spring within the displacer. The use of a Belleville washer and an annular spring having a u-shaped or c-shaped cross section were described.
FIG. 2 illustrates a graph 56 of a force-deflection curve 58 for a Belleville washer. Similarly, FIG. 3 illustrates a graph 60 of a force-deflection curve 62 for a u-shaped or c-shaped spring. The graphs 56, 60 illustrate that for both the Belleville washer and the c-shaped spring, small changes in deflection of the spring leads to large changes in the loads generated by the springs. Thus, with the Belleville washer and c-shaped spring, it is difficult to displace the springs and generate a force on the seal ring within a narrow load tolerance. Furthermore, because portions of the piston shrink with exposure to a cold environment, such shrinkage can cause small displacements on the Belleville washer and c-shaped spring. These displacements, while small, can drastically affect and change the loads generated by the springs. Maintaining a load on the seal ring within an acceptable tolerance level is difficult to achieve using the aforementioned springs.
Preferably, a spring used in a piston has a force-deflection relationship such that small changes in the deflection of the spring produce small changes in the load produced by the spring on the seal ring. The loads generated by the spring can therefore be obtained and held within a narrow range or tolerance. Preferably, the spring is a wave spring such as a spiral wave spring (Smalley Ring Co., Wheeling, Ill.) or at least one wavy washer (Smalley Ring Co., Wheeling, Ill.). For example, two wavy washers can be stacked vertically to form a dual wavy washer.
One embodiment of the invention includes a piston having a body with a circumferential groove, a seal ring and a wave spring. The groove includes a first groove wall and a second groove wall. The seal ring is located against the first groove wall in the body. The wave spring is located between the seal ring and the second groove wall where the wave spring axially loads the seal ring.
The piston can be used as a displacer within a refrigerator. The seal ring is preferably a split seal ring and includes at least one radial spring mounted within the seal ring to create a radial force within the seal ring. Preferably, the at least one radial spring includes a first radial spring and a second radial spring. The first radial spring includes a first opening and the second radial spring includes a second opening. The first radial spring and the second radial spring are mounted within the seal ring such that the first opening is located at approximately 180 degrees with respect to the second opening. In one embodiment, the piston includes a load ring mounted between the wave spring and the seal ring. The load ring distributes the axial load created by the wave spring onto the seal ring.
In another embodiment, the piston includes a sleeve mounted on the body where the sleeve forms the first groove wall of the circumferential groove and the body forming the second groove wall of the circumferential groove. The piston can also include a securing mechanism that secures the sleeve to the body such as a snap ring.
Another embodiment of the invention includes a refrigerator having a cylinder and a displacer mounted within the cylinder. The displacer includes a body with a circumferential groove, a seal ring and a spring. The groove includes a first groove wall and a second groove wall where the seal ring is located against the first groove wall in the body and the spring is located between the seal ring and the second groove wall, the spring axially loading the seal ring. The body can include a sleeve mounted on the body where the sleeve forms the first groove wall and the body forms the second groove wall. The spring can include a wave spring which axially loads the seal ring. The seal ring can include a split seal ring having at least one radial spring mounted within the split seal ring to create a radial force of the split seal ring against the cylinder.
An embodiment of the invention also relates to a method for securing a seal ring within a piston.